1. Vicariance underlies allopatric speciation in Snapping Shrimp
By Fred Singer

2. Allopatric speciation
1. Physical barrier causes isolation of one population from a second population.
2. Genetic divergence (usually in habitat use, resource use or mate choice).
3. Reproductive isolation

3. Physical isolation 1. Dispersal of a population to a new area.
2. Vicariance – Changing of a habitat by geological processes into two distinct habitats.

6. Snapping Shrimp - Alpheus

7. Nancy Knowlton et al.
Sister species (sibling species) – two species that are very closely related
Sister species usually are initially identified by similarities in morphology (morphospecies).
In the case of snapping shrimp, characters included:
Dorsal spots
Dorsal bands
Antenna color
Movable spines

8. Knowlton’s hypothesis
Vicariance (formation of the isthmus 3-4 million years ago) caused physical isolation of populations of these species, leading to allopatric speciation.
This resulted in seven pairs of sister species

9. Prediction 1
Sister species (based on morphology) should also be genetically more closely related to each other, than they are to other species
mtDNA (Cytochrome oxidase 1)
11 polymorphic allozymes

10. mtDNA tree

11. Conclusion
Based on the mtDNA, sister species identified by morphological characters are also closely related based on mtDNA analysis

12. Prediction 2
Because there was one vicariance event 3-4 million years ago, all seven sister species should have similar values separating the two sister species in relation to
mtDNA distance
allozyme distance
Similarities in mating behavior

13. Genetic Distances between sister species
Taxa
Allozymes (Nei’s D)
mtDNA (% divergence)
P1, C1
0.028
7.7
P2, C2
0.114
6.5
P3, C3
0.109
P4, C4
0.121
8.5
P5, C5
0.177
13.4
P6, C6
0.188
10.5
P7, C7
0.272
19.2

14. Behavioral compatibility measures
Taxa
Pairings
Passive
Contacts
Snaps
Aggressive
Overall compatibility
P1, C1
0.86
1.97
2.18
0.49
1.42
P2, C2
0.67
0.33
1.40
0.48
0.58
P3, C3
0.45
0.66
0.31
0.47
P4, C4
0.00
0.07
0.53
0.20
P5, C5
0.51
0.14
0.13
0.24
P6, C6
0.01
0.19
0.10
P7, C7

15. Conclusion
The data do not support prediction 2 of the vicariance hypothesis.
Rather some sister species (e.g. P1 and C1, and P2 and C2) are more similar to each other genetically and behaviorally than other sister species (such as P7 and C7).
These differences are consistent for all three measures (allozymes, mtDNA, behavior)
Thus P1 and C1 speciated more recently than P7 and C7.
Thus there was a different timing of speciation for each sister pair.

16. Huh?
Doesn’t vicariance predict that each sister pair should show similar levels of genetic and behavioral divergence?
We need to consider ecology!!
In particular, different species prefer different hangouts.

19. Conclusion
Vicariance has created the physical barrier for numerous speciation events in snapping shrimp.
We need to consider differences in ecology, in order to understand the sequence and timing of these speciation events.